It is known that there are, for all practical purposes, equal numbers of male and female determining spermatozoa at the locus of parental fertilization in the reproductive process in the vast majority of higher animals, including humans. The effect of this is that in general a nearly equal ratio of male and female decendants results from the reproductive processes of the population of the entire species.
This naturally-occurring ratio is disadvantageous in many instances and for certain purposes. For example, in the dairy industry, the female is generally desired for the production of milk, but in some cases a male is highly desired from a particularly good female. In the beef industry, the premium is on males.
In the rabbit industry, females are in demand because the female rabbit only is employed in the Ascheim-Zondeik Murray pregnancy test.
It has been known for some time that living spermatozoa can be concentrated to some extent according to their sex-determining character by electrophoresis. Thus, the migration of living sperm cells under the influence of a direct current electrical field can be observed by microscopic examination.
The electrophoretic method has been described by a number of workers in this field, e.g., V. N. Schroeder, Biol. Shur. 1. 24-29, (1932); 3, 465-476, (1934); 5, 657-689, 690-722, (1936); 6, 1235-1250, (1937); Bull. Acad. Sci. U.R.S.S., Ser. Biol., pp. 426-455, (1940); Compt. Rend. Acad. Sci. U.R.S.S. 26, 692-697 (1940); Z. Tierzuckt Zuchtungsbiol. 50, 1-15 and 16-23; (1941); Uspekhi Sovremennoi Biol. (Moscow) 28, 211-225, (1949); A. A. Siljander, Sbornik Trudov Zootek. Kaf. s.h. Skol. Kirov., pp. 148-166, (1936); and M. J. Gordon, Proc. Nat. Acad. Sci., 43, 913-918, (1957).
In the electrophoretic process the charged spermatozoa are attracted to an appropriately charged electrode. One of the problems with electrophoresis is that, when sufficient voltage is used to produce an adequately strong separating force, electrolysis also occurs. This causes an insulating hydrogen barrier to appear on the surface of the electrode which interfers with the operation of the process. Thus, relatively low voltages are required for the electrophoretic method, and the separating forces are relatively weak. Also, with the electrophoretic method, the spermatozoa are required to move comparatively long distances between the electrodes before separation or even concentration can be achieved. Thus, the weakness of the forces involved and the length of the distances required, render the electrophoretic method seriously slow. This is a major disadvantage because the life span of the spermatozoa is relatively short. The result is that the hitherto known electrophoretic methods only show a very modest degree of separation for treating times of sufficiently short duration to be practical. In fact, the entire operability of the electrophoretic method has been seriously challenged, (Hafs, H. D. and Boyd, L. J.; Am. Soc. Anm. Sci. Symposium Minutes 7/31/70 to 8/1/70, pp. 85- 97).
It is also known that the spermatozoa of the respective sexes have different sizes and weights, the female portion being larger and heavier. In addition, possibly due to its smaller size and weight, the male can move more rapidly. However, the nearly equal ratio of male and female offspring occurring in nature would indicate that, despite the increased motility of the male bearing spermatozoa, approximately equal numbers of males and females arrive simultaneously at the locus of parental fertilization. The differences between the sizes and weights of the spermatozoa of opposite sexes are due to differences in the amount of genetic material, that is, between the X-chromosome (female) and the Y-chromosome (male). From the available weight data, it is known that there is a probability of about 87.5% that a randomly chosen female-determining spermatozoa will be heavier than a randomly chosen male-determining spermatozoa. The actual weight and size differences, however, are not great. Thus, the difference between the extremes in weight which is reported in the literature is only about 6.2%. The size difference is comparable. Such differences are not sufficient to permit good separation of the respective sex-determining portions by simple filtration, sedimentation or centrifugation within periods of short enough duration to be practical.
Notwithstanding the disadvantages of the known methods for separating spermatozoa according to their sex-linked characteristics, the results have at times shown promise. For example, according to the publication of Gordon, Supra, using a known electrophoretic technique, out of 167 births in 31 litters of rabbits, the desired sex of the offsring was obtained 113 times. Thus, the involved success ratio was about 67.7%. However, the loss of spermatozoa was so great as to render the process of doubtful practical value. On the other hand, after conducting a much more extensive test by the electrophoretic method, Hafs was unable to demonstrate any degree of pre-selection, thus creating serious doubt as to the validity of the entire electrophoretic concept. The other known processes which rely on the size, weight, or motility differences of the spermatozoa have likewise shown only mild promise.
Accordingly, prior to the present invention, despite the demand for a process by which the sex of mammals could be safely and efficiently pre-selected, no known process has achieved the goal in a sufficiently certain or efficient manner to be considered practical.